U.S. Pat. Nos. 4,833,179 and 4,952,650 Young et al., assigned to the assignee of the present case, both incorporated by reference herein, relate to a method for suspension polymerization of a pressure-sensitive acrylatecopolymer bead having a glass transition temperature of 0.degree. C. or less. The method comprises making a monomer premix comprising an acrylic acid ester of non-tertiary alcohol, the alcohol having from 1 to 14 carbon atoms, with the average number of carbon atoms being about 4 to about 12, a polar monomer copolymerizable with the acrylic acid ester, a chain transfer agent, a free-radical initiator, and a modifier moiety selected from the group consisting of 2-polystyrylethyl methacrylate macromolecular monomers, reactive zinc salts and hydrophobic silicas. The premix is then combined with a water phase containing a sufficient amount of suspending agent to form a suspension. The suspension is concurrently agitated and polymerization of the polymer premix is permitted until polymer beads are formed. The polymer beads are then collected. The amount of the modifier moiety must be sufficient to render the copolymer bead non-agglomerating at room temperature for handling and transportation purposes.
The suspension beads are easily handleable and safely transportable. They are free flowing and do not agglomerate into unmanageable masses. They may be easily poured into tanks for transportation to distant manufacturing facilities. The beads are storage stable, and may be placed into storage tanks for long periods of time without undergoing physical or chemical degradation. The low Tg tacky polymer beads are prepared by an aqueous suspension polymerization technique utilizing modifier moiety to impart the handleability and transportability to the tacky suspension PSA beads. Although the beads disclosed in U.S. Pat. Nos. 4,833,179 and 4,952,650 are easily transportable they are not easily extrudable at low temperatures and must be extruded at 340.degree. F. to 380.degree. F. (171.degree. C. to 194.degree. C.). This is disadvantageous in that polymers are more susceptible to decomposition at such high temperatures.
The concept of inherent viscosity is important to the understanding of both extrudability and storage stability of suspension polymer beads. Inherent viscosity relates to the internal resistance to flow of a fluid. The inherent viscosity of a polymer or copolymer is related to a number of factors including molecular weight. Typically the higher the molecular weight of a polymer the higher its viscosity. Conversely, the lower the molecular weight of a polymer the lower its viscosity.
A low viscosity polymer is easier to coat due to its low internal resistance. However, a bead comprising a low viscosity polymer is more likely to clump during storage, due to its softness. This can cause problems during transportation and subsequent use since the beads will not easily pour when clumped together.
A high viscosity polymer is harder to coat due to its high internal resistance. Suspension beads comprising high viscosity polymers are less likely to be sticky and clump together. Thus, they have better storability. However, high viscosity polymers due to their lack of flow are difficult to pass through an extrusion coater unless high temperatures are employed. The use of high temperatures (i.e., about 350.degree.-380.degree. F.) to reduce the melt viscosity of the high viscosity polymer is disadvantageous in that decomposition of the polymer can occur at high temperatures.
Suspension polymerization in general is a well known method of polymerization in which the polymer formed is obtained as spherical beads or pearls. Other well known methods of polymerization include emulsion polymerization and solution polymerization. These methods both differ from suspension polymerization.
The polymer obtained via emulsion polymerization comprises fine particles of polymer stabilized by an emulsifier and dispersed uniformly in an aqueous phase. The dispersion obtained is often referred to as a "latex". No beads are obtained during emulsion polymerization.
Solution polymerization is a polymerization method which takes place in a solvent. The polymer obtained is also not in the form of a bead when solution polymerization is employed. Since no beads are obtained via solution or emulsion polymerization the balance between storage stability problems and extrusion problems are not encountered as they are in suspension polymerization.
Vinyl acetate has been used widely to copolymerize with acrylic esters via both solution and emulsion polymerization methods in order to form vinyl acrylic copolymer pressure sensitive adhesives. Such adhesives are less polar and adhere better to polyolefins and other nonpolar plastics, they usually exhibit a high tack, and may have a lower shear resistance as well as somewhat poorer aging properties than all acrylic adhesives. In addition, vinyl acetate is a less expensive monomer than acrylic esters. See Donatas Statas, Handbook of Pressure Sensitive Adhesive Technology, 2nd Ed. p 402, Van Nostrand Reinhold, 1989.
Christenson, U.S. Pat. No. 3,654,213 (issued Apr. 4, 1972) discloses pressure sensitive adhesives prepared via solution polymerization. The pressure sensitive adhesives are based on interpolymers of certain vinyl esters, of which vinyl acetate is one, and certain alkyl acrylates. The monomers are selected such that the interpolymers are compatible with liquid aliphatic hydrocarbons. The interpolymers of the art were only soluble in less desirable solvents of higher cost. Christenson's invention provided interpolymers which are compatible with less expensive aliphatic hydrocarbons.
Skouitchi et al. U.S. Pat. No. 4,069,123 discloses pressure sensitive adhesives based on acrylate ester homo and copolymers. The copolymers, which can be prepared via emulsion polymerization, are based on alkyl esters of acrylic and methacrylic acid and vinyl ester including vinyl acetate. Skoutichi relates to improved adhesive and cohesive properties obtained from the addition of specified quinone ultraviolet sensitizers.
Terpolymers of low T.sub.g acrylate/acrylic acid/vinyl acetate are known in the art and commercially available, such as Gelva.RTM. terpolymers available from Monsanto. These terpolymers are made from solvent and emulsion polymerization processes.
Thus, vinyl acetate has been used to modify the adhesive properties of various polymers prepared via emulsion or solution polymerizations which are not formed as beads. The use of vinyl acetate in the suspension polymerization of PSAs to yield suspension beads possessing storage stability and extrudability at low temperatures has never been taught or suggested by any reference.
A need thus exists for 1) a suspension bead which is easily extrudable at low temperatures (i.e., below about 340.degree. F., 170.degree. C.) in addition to being storage stable and 2) a method of forming such a bead. A need thus exists for a suspension bead having the advantages of both low inherent viscosity polymers (for ease of extrusion) and high inherent viscosity polymers (for good storage stability) without the disadvantages of either.
We have discovered such a suspension bead and a method of making such a suspension bead.